According to what I know now about Possessive Pronouns, we can to choose between one of the two following forms:
1) "This is your book" vs "This is yours"
2) "This is their book" vs "This is theirs"
3) "This is her book" vs "This is hers"
4) "This is our book" vs "This is ours"
5) "This is my book" vs "This is mine"
But I'm confused about two Possessive Pronouns: Its & his
Can I say (on the way that was mentioned above):
a) "This is his book" vs "This is his."
b) "This is its part" vs "This is its."